1. Field of the Invention
The present invention relates to a biaxially oriented polypropylene film realizing an excellent balance between a high withstand voltage and an element-winding suitability, the film thickness thereof being very thin.
The present invention claims priority on Japanese Patent Application No. 2006-297250 filed on Nov. 1, 2006, and Japanese Patent Application No. 2007-272616 filed on Oct. 19, 2007, the contents of which are incorporated herein by reference.
2. Description of the Related Art
A biaxially oriented polypropylene film is widely used for electrical application due to its superior electronic properties, such as, for example, withstand voltage and dielectric loss properties, to those of other plastic films. Particularly, a biaxially oriented polypropylene film is widely used as a dielectric film for a capacitor and its demand is significantly increasing. In recent years, various electric equipment have been made to be equipped with an inverter, and thereby the need for downsizing and increasing capacitance of the capacitor has been further increased. In order to meet such marketing needs, it is required that a biaxially oriented polypropylene film be further thinned, while making the biaxially oriented polypropylene film improved in terms of the withstand voltage and processing suitability for winding elements.
While demand for a capacitor film is increasing, the need for a capacitor with further high withstand voltage is particularly intense in the market. Patent Document 1 (Japanese Patent Publication No. 3,654,540) discloses that the withstand voltage is improved by decreasing the ash content in a resin. Also, Patent Document 2 (Japanese Patent Publication No. 3,791,038) discloses that the withstand voltage is improved by making a polypropylene resin have high stereoregularity and crystallinity.
However, such a high stereoregularity as disclosed in Patent Document 2 causes deterioration of the drawability and causes the film to be easily torn during drawing, which is unfavorable for manufacturing a oriented thin film.
On the other hand, the surface of such a film for a capacitor is required to be finely roughened in moderation, so as to improve processing suitability for winding elements at the time of producing a capacitor and sliding properties of the film at the time of processing the film, and also so as to improve oil impregnation properties when the film is applied to an oil-impregrated capacitor. Patent Document 3 (Japanese Unexamined Patent Application, First Publication No. S51-63500) discloses a method for finely roughening the surface of a film.
As a method for finely roughening the surface, there is a method in which a sheet with β-form crystal is drawn. Non-Patent Document 1 (“Introduction to high polymer under polarizing microscope” attributed to Hiroshi Kuriya, published by AGNE GIJYUTSU CENTER, page 131, 2001) discloses that polypropylene resins generally have crystal polymorphism such as α-crystal β-crystal, or the like. The β-crystal has a lower density, a lower melting point, and other different physical properties, in comparison to those of the α-crystal. When a molten polypropylene resin is crystallized within a particular temperature range, the β-crystal is generated. When this β-form is drawn at the vicinity of the melting point thereof, a β-spherulite transfers to an α-spherulite with a density different from that of β-crystal. The density-difference between these crystals generates fine unevenness (plural fine recesses and protrusions) on the film surface. When the surface is roughened by this method, impurities such as additives are not required to be added to the resin. Accordingly, this method is advantageous in forming extremely fine unevenness without deteriorating electric characteristics thereof.
When the surface is roughened with the β-crystal, the formation of the β-crystal under control during the processing for a sheet is technically important. As for the formation of the β-crystal, Patent Document 4 (Japanese Laid-Open Patent Application No. 2004-2655), Patent Document 5 (Japanese Laid-Open Patent Application No. 2004-175932), and Patent Document 6 (Japanese Laid-Open Patent Application No. 2005-89683), for example, disclose that a sheet with a high β-crystal proportion can be produced from a polypropylene resin obtained by polymerization using a particular catalyst, the polypropylene resin having a certain range of melt flow rate, molecular weight, and molecular weight distribution.
In addition, as for the preparation of an oriented polypropylene film with a roughened surface, Patent Document 7 (Japanese Patent Publication No. 3,508,515) discloses a technique in which a film with a roughened surface having a particular range of center line average roughness (Ra) is manufactured by using a polypropylene raw resin with a particular stereoregularity to control the content of β-crystal in a cast raw sheet to a particular content or more.
However, as disclosed in Patent Document 8 (Japanese Unexamined Patent Application, First Publication No. H9-270364) and Patent Document 9 (Japanese Laid-Open Patent Application No. 2002-105224), such a surface roughening generally has the disadvantage of inviting a degradation of electric breakdown voltage, although the surface roughening is essential to enhance processing suitability.
On the other hand, a capacitor downsized without changing capacitance is required, and therefore needs for an extremely thin film have been increasing. In order to obtain such an extremely thin film, the enhancement of the drawability of a raw resin is an essential matter. In order to realize such an enhancement, increasing polydispersity index of the raw resin is useful.
However, such a widening of the molecular weight distribution generally causes deterioration of crystallinity of the resin, as a result of which the withstand voltage of a capacitor deteriorates.
Patent Documents 8 and 9 each disclose a method for obtaining a film with an element-winding suitability and withstand voltage. According to the methods disclosed in Patent Documents 8 and 9, the element-winding suitability and the withstand voltage are provided by adjusting the center line average roughness (Ra) to be within a particular range. However, the element-winding suitability and withstand voltage realized by such a technique have not yet met the needs of the market, and also thinning of the film has not yet been sufficient.
Thus, realization of the high withstand voltage (realization of smoothed surface and high crystallinity) (1) is incompatible with realization of the processing suitability for producing a capacitor (realization of roughened surface) (2) and realization of the extremely thinned film (widening of the molecular weight distribution) (3), the above (1) to (3) being required by the market, and a capacitor film achieving them all has not been provided till now.